This fixes#7157. It doesn't change the way aws_ami works
```
make testacc TEST=./builtin/providers/aws
TESTARGS='-run=TestAccAWSAMICopy'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /vendor/)
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSAMICopy
-timeout 120m
=== RUN TestAccAWSAMICopy
--- PASS: TestAccAWSAMICopy (479.75s)
PASS
ok github.com/hashicorp/terraform/builtin/providers/aws 479.769s
```
AWS provides three different ways to create AMIs that each have different
inputs, but once they are complete the same management operations apply.
Thus these three resources each have a different "Create" implementation
but then share the same "Read", "Update" and "Delete" implementations.
